# 1 Probability, Conditional Probability and Bayes Formula

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ISyE8843A, Brani Vidakovic Handout 1

## Probability, Conditional Probability and Bayes Formula

The intuition of chance and probability develops at very early ages.1 However, a formal, precise definition of the probability is elusive.

If the experiment can be repeated potentially infinitely many times, then the probability of an event can be defined through relative frequencies. For instance, if we rolled a die repeatedly, we could construct a frequency distribution table showing how many times each face came up. These frequencies (ni) can be

expressed as proportions or relative frequencies by dividing them by the total number of tosses n : fi = ni/n.If we saw six dots showing on 107 out of 600 tosses, that face’s proportion or relative frequency is

f6 = 107/600 = 0.178As more tosses are made, we “expect” the proportion of sixes to stabilize around 16.

Famous Coin Tosses: Buffon tossed a coin 4040 times. Heads appeared 2048 times. K. Pearson tossed a coin 12000 times and 24000 times. The heads appeared 6019 times and 12012, respectively. For these three tosses the relative frequencies of heads are 0.5049, 0.5016,and 0.5005.

What if the experiments can not be repeated? For example what is probability that Squiki the guinea pig survives its first treatment by a particular drug. Or “the experiment” of you taking ISyE8843 course in Fall 2004. It is legitimate to ask for the probability of getting a grade of anA. In such cases we can define probabilitysubjectivelyas a measure of strength of belief.

Figure 1: A gem proof condition 1913 Liberty Head nickel, one of only five known and the finest of the five. Collector Jay Parrino of Kansas City bought the elusive nickel for a record \$1,485,000, the first and only time an American coin has sold for over \$1 million.

Tutubalin’s Problem. In a desk drawer in the house of Mr Jay Parrino of Kansas City there is a coin, 1913 Liberty Head nickel. What is the probability that the coin is heads up?

Thesymmetryproperties of the experiment lead to the classical definition of probability. An ideal die is symmetric. All sides are “equiprobable”. The probability of 6, in our example is a ratio of the number of favorable outcomes (in our example only one favorable outcome, namely, 6 itself) and the number of all possible outcomes, 1/6.2

1Piaget, J. and Inhelder B.The Origin of the Idea of Chance in Children, W. W. Norton & Comp., N.Y.

2This definition is attacked by philosophers because of the fallacy calledcirculus vitiosus. One defines the notion of probability

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(Frequentist) An event’sprobabilityis the proportion of times that we expect the event to occur, if the experiment were repeated a large number of times.

(Subjectivist) A subjectiveprobabilityis an individual’s degree of belief in the occurrence of an event.

(Classical) An event’s probabilityis the ratio of the number of favorable outcomes and possible outcomes in a (symmetric) experiment.

Term Description Example

Experiment Phenomenon where outcomes are

un-certain Single throws of a six-sided die

Sample space Set of all outcomes of the experiment

S =

{1,2,3,4,5,6}, (1,2,3,4,5, or 6 dots show)

Event A collection of outcomes; a subset of S

A = {3} (3 dots show), B = {3,4,5, or6}(3, 4, 5, or 6 dots show) or ’at least three dots show’

Probability A number between 0 and 1 assigned to

an event. P(A) =

1

6. P(B) = 46.

Sure eventoccursevery timean experiment is repeated and has the probability 1. Sure event is in fact the sample spaceS.

An event thatneveroccurs when an experiment is performed is calledimpossible event. The probability of an impossible event, denoted usually byis 0.

For any event A, the probability that A will occur is a number between 0 and 1, inclusive:

0≤P(A)1, P() = 0, P(S) = 1.

Theintersection (product)A·B of two events A and B is an event that occurs if both events AandB occur. The key word in the definition of the intersection isand.

In the case when the eventsAandBare independent the probability of the intersection is the product of probabilities:P(A·B) =P(A)P(B).

Example:The outcomes of two consecutive flips of a fair coin are independent events.

Events are said to be mutually exclusiveif they have no outcomes in common. In other words, it is impossible that both could occur in a single trial of the experiment. For mutually exclusive events holds P(A·B) =P(∅) = 0.

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In the die-toss example, events A = {3} andB = {3,4,5,6} are not mutually exclusive, since the outcome{3}belongs to both of them. On the other hand, the eventsA={3}andC={1,2}are mutually exclusive.

The unionA∪Bof two eventsAandBis an event that occurs if at least one of the eventsAorBoccur. The key word in the definition of the union isor.

For mutually exclusive events, the probability that at least one of them occurs is P(A∪C) =P(A) +P(C)

For example, if the probability of eventA ={3}is 1/6, and the probability of the eventC = {1,2}is 1/3,then the probability of A or C is

P(A∪C) =P(A) +P(C) = 1/6 + 1/3 = 1/2.

Theadditivityproperty is valid for any number of mutually exclusive eventsA1, A2, A3, . . .:

P(A1∪A2∪A3∪. . .) =P(A1) +P(A2) +P(A3) +. . . What isP(A∪B)if the eventsAandBare not mutually exclusive.

For any two eventsAandB, the probability that eitherAorBwill occur is given by theinclusion-exclusionrule

P(A∪B) =P(A) +P(B)−P(A·B)

If the eventsAabdB are exclusive, thenP(A·B) = 0, and we get the familiar formulaP(A∪B) = P(A) +P(B).

The inclusion-exclusion rule can be generalized to unions of arbitrary number of events. For example, for three eventsA, BaandC, the rule is:

P(A∪B∪C) =P(A) +P(B) +P(C)−P(A·B)−P(A·C)−P(B·C) +P(A·B·C).

For every event defined onS, we can define a counterpart-event called itscomplement. The complement Acof an eventAconsists of all outcomes that are inS, but arenotinA.The key word in the definition of

an complement isnot.In our example,Acconsists of the outcomes:{1,2,3,4,5}.

The eventsAandAcare mutually exclusive by definition. Consequently,

P(A∪Ac) =P(A) +P(Ac)

Since we also know from the definition ofActhat it includes all the events in the sample space,S, that

are not inA, so

P(A) +P(Ac) =P(S) = 1 For any complementary eventsAandAc,

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These equations simplify solutions of some probability problems. If P(Ac) is easier to calculate than

P(A), thenP(Ac)and equations above let us obtainP(A)indirectly.

This and some other properties of probability are summarized in table below.

Property Notation

If event S willalwaysoccur, its probability is 1. P(S) = 1 If eventwillneveroccur, its probability is 0. P() = 0 Probabilities are always between 0 and 1, inclusive 0≤P(A)1 IfA, B, C, . . . are all mutually exclusive thenP(A∪B∪

C . . .)can be found by addition.

P(A∪B ∪C . . .) = P(A) + P(B) +P(C) +. . .

IfAandB are mutually exclusive thenP(A∪B)can be

found by addition. P(A∪B) =P(A) +P(B)

The generaladdition rulefor probabilities P(A∪B) = P(A) +P(B)− P(A·B)

SinceAandAcare mutually exclusive and between them

include all possible outcomes,P(A∪Ac)is 1.

P(A∪Ac) =P(A) +P(Ac) =

P(S) = 1,and P(Ac) = 1 P(A)

## Conditional Probability and Independence

A conditional probability is the probability of one event if another event occurred. In the “die-toss” example, the probability of event A, three dots showing, is P(A) = 16 on a single toss. But what if we know that event B, at least three dots showing, occurred? Then there are only four possible outcomes, one of which is A. The probability of A = {3}is 1

4, given thatB = {3,4,5,6}occurred. The conditional

probability ofAgiven Bis writtenP(A|B).

P(A|B) = P(A·B)

P(B)

EventAisindependentofBif the conditional probability ofAgivenBis the same as the unconditional probability ofA. That is, they are independent if

P(A|B) =P(A)

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The probability that two eventsAandB will both occur is obtained by applying the

multiplication rule:

P(A·B) =P(A)P(B|A) =P(B)P(A|B)

whereP(A|B) (P(B|A))means the probability ofAgivenB(B givenA). For independent events only, the equation in the box simplifies to

P(A·B) =P(A)P(B).

ProveP(A1A2. . . An) =P(A1|A2. . . An)P(A2|A3. . . An) . . . P(An−1|An)P(An).

Example: Let the experiment involves a random draw from a standard deck of 52 playing cards. Define eventsA andB to be ”the card isand “the card is queen”. Are the eventsA andB independent? By definition,P(A&B) =P(Q) = 521.This is the product ofP(♠) = 1352andP(Q) = 524,andAandBin question are independent. The intuition barely helps here. Pretend that from the original deck of cards2 is excluded prior to the experiment. Now the eventsAandB become dependent since

P(A)·P(B) = 13 51 · 4 51 6= 1 51 =P(A&B).

The multiplication rule tells us how to find probabilities for composite event (A·B). The probability of (A·B) is used in the generaladdition rulefor finding the probability of (A∪B).

Rule Notation

Definitions

Theconditional probabilityofAgivenBis the probability

of eventA, if eventB occurred. P(A|B) A isindependentofB if the conditional probability ofA

givenB is the same as the unconditional probability ofA. P(A|B) =P(A)

Multiplication rule:

The generalmultiplication rulefor probabilities P(A ·B) = P(A)P(B|A) = P(B)P(A|B)

Forindependent eventsonly, the multiplication rule is

sim-plified. P(A·B) =P(A)P(B)

## Pairwise and Global Independence

If three eventsA, B,andCare such that any of the pairs are exclusive, i.e.,AB=,AC =orBC =, then the events are mutually exclusive, i.e, ABC = . In terms of independence the picture is different. Even if the events are pairwise independent for all three pairs A, B; A, C; and B, C, i.e., P(AB) = P(A)P(B), P(AC) = P(A)P(C),andP(BC) = P(B)P(C),they may be dependent in the totality, P(ABC)6=P(A)P(B)P(C).

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The four sides of a tetrahedron (regular three sided pyramid with 4 sides consisting of isosceles triangles) are denoted by 2, 3, 5 and 30, respectively. If the tetrahedron is “rolled” the number on the basis is the outcome of interest. Let the three events areA-the number on the base is even, B-the number is divisible by 3, andC-the number is divisible by 5. The events are pairwise independent, but in totality dependent.

Algebra is clear here, but what is the intuition. The “trick” is that the eventsAB,AC,BC andABC all coincide. That isP(A|BC) = 1althoughP(A|B) =P(A|C) =P(A).

The concept of independence (dependence) is not transitive. At first glance that may seem not correct, as one may argue as follows, “If event A depends on B, and event B depends on C, then event A should depend on the event C.” A simple exercise proves the above statement wrong.

Take a standard deck of 52 playing cards and replace the♣Qwith♦Q.Thus the deck now still has 52 cards, two♦Qand no♣Q. From such a deck draw a card at random and consider three events: A-the card is queen, B-the card is red, and C- the card is♥.It ie easy to see that A andB are dependent sinceP(A&B) = 3/52 6= P(A)·P(B) = 4/52·27/52.The events B andCare dependent as well since the eventC is contained inB, andP(BC) = P(C) 6= P(B) ·P(C).However, the events A and C are independent, since P(AC) = P(♥Q) = 1/52 =P(A)P(C) = 13/53·4/52.

## Total Probability

EventsH1, H2, . . . , Hnform a partition of the sample spaceSif

(i) They are mutually exclusive (Hi·Hj =∅, i6=j) and

(ii) Their union is the sample spaceS, Sni=1Hi =S.

The eventsH1, . . . , Hn are usually calledhypothesesand from their definition follows thatP(H1) +

· · ·+P(Hn) = 1 (=P(S)).

Let the event of interestAhappens under any of the hypothesesHiwith a known (conditional)

probabil-ityP(A|Hi).Assume, in addition, that the probabilities of hypothesesH1, . . . , Hnare known. ThenP(A)

can be calculated using thetotal probability formula.

Total Probability Formula.

P(A) =P(A|H1)P(H1) +· · ·+P(A|Hn)P(Hn).

The probability of A is the weighted average of the conditional probabilities P(A|Hi) with weights

P(Hi).

Stanley.Stanley takes an oral exam in statistics by answering 3 questions from an examination card drawn at random from the set of 20 cards. There are 8 favorable cards (Stanley knows answers on all 3 questions). Stanley will get a gradeAif he answers all 3 questions. What is the probability for Stanley to get anAif he draws the card

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(b) second (c) third?

Solution:Denote withAthe event that Stanley draws a favorable card (and consequently gets anA). (i) If he draws the card first, then clearlyP(A) = 8/20 = 2/5.

(ii) If Stanley draws the second, then one card was taken by the student before him. That first card taken might have been favorable (hypothesis H1) or unfavorable (hypothesis H2). Obviously, the hypotheses H1 andH2 partition the sample space since no other type of cards is possible, in this context. Also, the

probabilities ofH1 andH2 are 8/20 and 12/20, respectively. Now, after one card has been taken Stanley

draws the second. IfH1 had happened, probability ofAis 7/19, and ifH2 had happened, the probability ofAis 8/19. Thus, P(A|H1) = 7/19 andP(A|H2) = 8/19.By the total probability formula,P(A) =

7/19·8/20 + 8/19·12/20 = 8/20 = 2/5.

(iii) Stanley has the same probability of getting an A after two cards have been already taken. The hypotheses areH1={both cards taken favorable},H2={exactly one card favorable}, andH3={none of the

cards taken favorable}.P(H1) = 8/20·7/19, P(H3) = 12/20·11/19.andP(H2) = 1−P(H1)−P(H3).

Next,P(A|H1) = 6/18, P(A|H2) = 7/18,andP(A|H3) = 8/18.Therefore,P(A) = 6/18·7/19·8/20+ 7/18·...+ 8/18·11/19·12/20 = 12/20.

Moral:Stanley’s lack on the exam does not depend on the order in drawing the examination card.

Two-headed coin Out of 100 coins one has heads on both sides. One coin is chosen at random and flipped two times. What is the probability to get

(a) two heads? (b) two tails?

Solution:

(a) LetAbe the event that two heads are obtained. Denote byH1 the event (hypothesis) that a fair coin was chosen. The hypothesisH2 =H1cis the event that the two-headed coin was chosen.

P(A) =P(A|H1)P(H1) +P(A|H2)P(H2) = 1/4·99/100 + 1·1/100 = 103/400 = 0.2575.

(b) Exercise. [Ans. 0.2475]

## Bayes’ Formula

Recall that multiplication rule claims:

P(AH) =P(A)P(H|A) =P(H)P(A|H). This simple identity is the essence ofBayes’ Formula.

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Bayes Formula. Let the event of interestAhappens under any of hypothesesHiwith

a known (conditional) probabilityP(A|Hi).Assume, in addition, that the

probabili-ties of hypothesesH1, . . . , Hnare known (priorprobabilities). Then the conditional

(posterior) probability of the hypothesis Hi, i = 1,2, . . . , n, given that event A

happened, is P(Hi|A) = P(A|Hi)P(Hi) P(A) , where P(A) =P(A|H1)P(H1) +· · ·+P(A|Hn)P(Hn).

Assume that out of N coins in a box, one has heads at both sides. Such “two-headed” coin can be purchased in Spencer stores. Assume that a coin is selected at random from the box, and without inspecting it, flippedktimes. Allktimes the coin landed up heads. What is the probability that two headed coin was selected?

Denote withAkthe event that randomly selected coin lands heads upktimes. The hypotheses areH1-the

coin is two headed, andH2 the coin is fair. It is easy to see thatP(H1) = 1/N andP(H2) = (N 1)/N. The conditional probabilities areP(Ak|H1) = 1for anyk, andP(Ak|H2) = 1/2k.

By total probability formula,

P(Ak) = 2 k+N1 2kN , and P(H1|Ak) = 2k 2k+N 1.

ForN = 1,000,000andk= 1,2, . . . ,30the graph of posterior probabilities is given in Figure 2 It is interesting that our prior probabilityP(H1) = 0.000001jumps to posterior probability of 0.9991,

after observing 30 heads in a row. The matlab code bayes1 1.m producing Figure 2 is given in the Programs/Codes on the GTBayes Page.

Prosecutor’s FallacyThe prosecutor’s fallacy is a fallacy commonly occurring in criminal trials but also in other various arguments involving rare events. It consists of subtle exchanging ofP(A|B)forP(B|A). A zealous prosecutor has collected an evidence, say fingerprint match, and has an expert testify that the probability of finding this evidence if the accused were innocent is tiny. The fallacy is committed the prosecutor proceeds to claim that the probability of the accused being innocent is comparably tiny.

Why is this incorrect? Suppose there is a one-in-a-million chance of a match given that the accused is innocent. The prosector deduces that means there is only a one-in-a-million chance of innocence. But in a community of 10 million people, one expects 10 matches, and the accused is just one of those ten. That would indicate only a one-in-ten chance of guilt, if no other evidence is available.

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0 5 10 15 20 25 30 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 number of flips posterior probability

Figure 2: Posterior probability of the “two-headed” coin forN = 1,000,000ifkheads appeared. The fallacy thus lies in the fact that the a prioriprobability of guilt is not taken into account. If this probability is small, then the only effect of the presented evidence is to increase that probability somewhat, but not necessarily dramatically.

As a real-life example, consider the case of Sally Clark , who was accused in 1998 of having killed her first child at 11 weeks of age, then conceived another child and killed it at 8 weeks of age. The prosecution had expert witness testify that the probability of two children dying from sudden infant death syndrome is about 1 in 73 million. To provide proper context for this number, the probability of a mother killing one child, conceiving another and killing that one too, should have been estimated and compared to the 1 in 73 million figure, but it wasn’t. Ms. Clark was convicted in 1999, resulting in a press release by the UK Royal Statistical Society which pointed out the mistake. Sally Clark’s conviction was eventually quashed on other grounds on appeal on 29th January 2003.

In another scenario, assume a burglary has been committed in a town, and 10,000 men in the town have their fingerprints compared to a sample from the crime. One of these men has matching fingerprint, and at his trial, it is testified that the probability that two fingerprint profiles match by chance is only 1 in 20,000. This does not mean the probability that the suspect is innocent is 1 in 20,000. Since 10,000 men were taken fingerprints, there were 10,000 opportunities to find a match by chance; the probability that there was at least one fingerprint match is1¡100000 ¢·¡200001 ¢0·¡1200001 ¢10000, about 39% – considerably more than 1 in 20,000. (The probability that exactly one of the 10,000 men has a match is¡100001 ¢·¡200001 ¢1·¡1200001 ¢9999 or about 30%, which certainly casts a reasonable doubt.)

Return to our example withN coins. Assume that out ofN = 1000000coins in a box, one has heads at both sides. Such “two-headed” coin is “guilty.” Assume that a coin is selected at random from the box, and without inspecting it, flippedk = 15times. All k = 15times the coin landed up heads. Based on this evidence the “prosecutor” claims the selected coin is guilty since if it was “innocent,” observedk= 15 heads in a row will appear with probability 2115 0.00003.But the probability that the “guilty” coin was

really selected is 1

1+999999/215 0.03 and prosecutor is accusing an “innocent” coin with probability of

approximately 0.97.

In legal terms, the prosecutor is operating in terms of a presumption of guilt, something which is contrary to the normal presumption of innocence where a person is assumed to be innocent unless found guilty.

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Two Masked Robbers.Two masked robbers try to rob a crowded bank during the lunch hour but the teller presses a button that sets off an alarm and locks the front door. The robbers realizing they are trapped, throw away their masks and disappear into the chaotic crowd. Confronted with 40 people claiming they are innocent, the police gives everyone a lie detector test. Suppose that guilty people are detected with probability 0.85 and innocent people appear to be guilty with probability 0.08. What is the probability that Mr. Smith was one of the robbers given that the lie detector says he is?

Guessing. Subject in an experiment are told that either a red or a green light will flash. Each subject is to guess which light will flash. The subject is told that the probability of a red light is 0.7, independent of guesses. Assume that the subject is a probability matcher- that is , guesses red with probability .70 and green with probability .30.

(i) What is the probability that the subject guesses correctly?

(ii) Given that a subject guesses correctly, what is the probability that the light flashed red?

False Positives. False positives are a problem in any kind of test: no test is perfect, and sometimes the test will incorrectly report a positive result. For example, if a test for a particular disease is performed on a patient, then there is a chance (usually small) that the test will return a positive result even if the patient does not have the disease. The problem lies, however, not just in the chance of a false positive prior to testing, but determining the chance that a positive result is in fact a false positive. As we will demonstrate, using Bayes’ theorem, if a condition is rare, then the majority of positive results may be false positives, even if the test for that condition is (otherwise) reasonably accurate.

Suppose that a test for a particular disease has a very high success rate:

if a tested patient has the disease, the test accurately reports this, a ’positive’, 99% of the time (or, with probability 0.99), and

if a tested patient does not have the disease, the test accurately reports that, a ’negative’, 95% of the time (i.e. with probability 0.95).

Suppose also, however, that only 0.1% of the population have that disease (i.e. with probability 0.001). We now have all the information required to calculate the probability that, given the test was positive, that it is a false positive.

LetD be the event that the patient has the disease, andP be the event that the test returns a positive result. The probability of a true positive is

P(D|P) = 0.99×0.001

0.99×0.001 + 0.05×0.999 0.019, and hence the probability of a false positive is about (1 - 0.019) = 0.981.

Despite the apparent high accuracy of the test, the incidence of the disease is so low (one in a thousand) that the vast majority of patients who test positive (98 in a hundred) do not have the disease. Nonetheless, this is 20 times the proportion before we knew the outcome of the test! The test is not useless, and re-testing may improve the reliability of the result. In particular, a test must be very reliable in reporting a negative result when the patient does not have the disease, if it is to avoid the problem of false positives. In mathematical terms, this would ensure that the second term in the denominator of the above calculation is small, relative to the first term. For example, if the test reported a negative result in patients without the disease with probability 0.999, then using this value in the calculation yields a probability of a false positive of roughly 0.5.

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Multiple Choice. A student answers a multiple choice examination question that has 4 possible answers. Suppose that the probability that the student knows the answer to the question is 0.80 and the probability that the student guesses is 0.20. If student guesses, probability of correct answer is 0.25.

(i) What is the probability that the fixed question is answered correctly?

(ii) If it is answered correctly what is the probability that the student really knew the correct answer.

Manufacturing Bayes. Factory has three types of machines producing an item. Probabilities that the item is I quality f it is produced oni-th machine are given in the following table:

machine probability of I quality

1 0.8

2 0.7

3 0.9

The total production is done 30% on type I machine, 50% on type II, and 20% on type III. One item is selected at random from the production.

(i) What is the probability that it is of I quality?

(ii) If it is of first quality, what is the probability that it was produced on the machine I?

Two-headed coin. 4 One out of 1000 coins has two tails. The coin is selected at random out of these 1000 coins and flipped 5 times. If tails appeared all 5 times, what is the probability that the selected coin was ‘two-tailed’?

Kokomo, Indiana. In Kokomo, IN, 65% are conservatives, 20% are liberals and 15% are independents. Records show that in a particular election 82% of conservatives voted, 65% of liberals voted and 50% of independents voted.

If the person from the city is selected at random and it is learned that he/she did not vote, what is the probability that the person is liberal?

Inflation and Unemployment. Businesses commonly project revenues under alternative economic scenar-ios. For a stylized example, inflation could be high or low and unemployment could be high or low. There are four possible scenarios, with the assumed probabilities:

Scenario Inflation Unemployment Probability

1 high high 0.16

2 high low 0.24

3 low high 0.36

4 low low 0.24

(i) What is the probability of high inflation?

(ii) What is the probability of high inflation if unemployment is high? (iii) Are inflation and unemployment independent?

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Information Channel. One of three words AAAA, BBBB, and CCCC is transmitted via an information channel. The probabilities of these words are 0.3, 0.5, and 0.2, respectively. Each letter is transmitted independently of the other letters and it is received correctly with probability 0.6. Since the channel is not perfect, the letter can change to one of the other two letters with equal probabilities of 0.2. What is the probability that word AAAA had been submitted if word ABCA was received.

Jim Albert’s Question. An automatic machine in a small factory produces metal parts. Most of the time (90% by long records), it produces 95% good parts and the remaining have to be scrapped. Other times, the machine slips into a less productive mode and only produces 70% good parts. The foreman observes the quality of parts that are produced by the machine and wants to stop and adjust the machine when she believes that the machine is not working well. Suppose that the first dozen parts produced are given by the sequence

s u s s s s s s s u s u

wheres– satisfactory andu– unsatisfactory. After observing this sequence, what is the probability that the machine is in its good state? If the foreman wishes to stop the machine when the probability of “good state” is under 0.7, when should she stop?

Medical Doctors and Probabilistic Reasoning. The following problem was posed by Casscells, Schoen-berger, and Grayboys (1978) to 60 students and staff at Harvard Medical School:If a test to detect a disease whose prevalence is 1/1000 has a false positive rate of 5%, what is the chance that a person found to have a positive result actually has the disease, assuming you know nothing about the person’s symptoms or signs? Assuming that the probability of a positive result given the disease is 1, the answer to this problem is approximately 2%. Casscells et al. found that only 18% of participants gave this answer. The modal response was 95%, presumably on the supposition that, because an error rate of the test is 5%, it must get 95% of results correct.

Let’s Make a Deal. In the popular television game showLet’s Make a Deal,Monty Hall is the master of ceremonies. At certain times during the show, a contestant is allowed to choose one of three identical doors A, B, C, behind only one of which is a valuable prize (a new car). After the contestant picks a door (say, door A), Monty Hall opens another door and shows the contestant that there is no prize behind that door. (Monty Hall knows where the prize is and always chooses a door where there is no prize.) He then asks the contestant whether he or she wants to stick with their choice of door or switch to the remaining unopened door. Should the contestant switch doors? Does it matter?

A federalist paper resolved? Suppose that a work, the author of which is known to be either Madson or Hamilton, contains a certain key phrase. Suppose further that this phrase occurs in 60% of the papers known to have been written by Madison, but in only 20% of those by Hamilton. Finally, suppose a historian gives subjective probability .3 to the event that the author is Madison (and consequently .7 to the complementary event that the author is Hamilton). Compute the historian’s posterior probability for the event that the author is Madison.

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## References

 Batt, J. Stolen Innocence: A Mother’s Fight for Justice. The Authorised Story of Sally Clark, Ebury Press.

 Barbeau, E. (1993). The Problem of the Car and Goats,CMJ,24:2, p. 149

 Casscells, W., Schoenberger, A., and Grayboys, T. (1978). Interpretation by physicians of clinical laboratory results.New England Journal of Medicine,299, 999-1000.

 Gillman, L. (1992). The Car and the Goats,AMM99:1, p. 3

## Figure Figure 1: A gem proof condition 1913 Liberty Head nickel, one of only five known and the finest of the five. p.1 Figure 2: Posterior probability of the “two-headed” coin for N = 1, 000, 000 if k heads appeared. p.9

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